Bash + curl + jq: 73 bytes

Shortest answer that doesn't use an xkcd-specific library. jq is a tool for manipulating json objects in the shell, and it comes complete with a parsing language to do that.

curl -Ls xkcd.com/$1/info.0.json|jq -r 'if.num==859then.num.a else.alt end'

Expansion below:

curl -Ls - Query, but feel free to redirect (in this case to the https site) and give no unrelated output.

xkcd.com/$1/info.0.json - Shamelessly stolen from another answer.

|jq -r - Run jq in "raw output" mode on the following command.

    .num == 859
then
    .num.a # This fails because you can't get the key 'a' from a property that's an integer
else
    .alt # And this pulls out the 'alt' key from our object.
end

Bash + curl + jq: 73 66 bytes

Shortest answer that doesn't use an xkcd-specific library. jq is a tool for manipulating json objects in the shell, and it comes complete with a parsing language to do that.

curl -Ls xkcd.com/$1/info.0.json|jq -r 'if.num==859then.num.a else.alt end'

curl -Ls xkcd.com/$1/info.0.json|jq -r '(.num!=859//.[9]|not)//.alt'

Expansion below:

curl -Ls - Query, but feel free to redirect (in this case to the https site) and give no unrelated output.

xkcd.com/$1/info.0.json - Shamelessly stolen from another answer.

|jq -r - Run jq in "raw output" mode on the following command.

    .num == 859
then
    .num.a # This fails because you can't get the key 'a' from a property that's an integer
else
    .alt # And this pulls out the 'alt' key from our object.
end

Now the script has been re-worked to use // which is the equivalent of a or b in python, and we use a |not to make any true value be considered false, so the second // can print .alt